Search PubMed⌕ Search

Biomedical subjects

M A Rea

Publications and source records attributed to M A Rea.

At least 55 records · Page 3Linked to original sources
PubMed1990

Ultrasound and the blood-brain barrier.

High intensity focused ultrasound was employed to modify the permeability of the normal feline and canine blood-brain barrier (BBB) to a circulating vital dye--Evans blue (EB). The threshold doses (W sec/cm2) for focally increasing the permeability of the BBB in white matter (WM) and gray matter (GM) were as follows: internal capsule (WM)--340 to 680; thalamus (GM)--approximately 1326; and caudate nucleus (GM)--2284 to 2952. In the presence of supralesioning doses of ultrasound, the cross sectional area occupied by the EB was consistently greater than that of the attendant nonhemorrhagic lesion--thus suggesting that BBB changes may be inducible at sublesioning doses. These findings, in conjunction with those of others, suggest that high intensity focused ultrasound may have a role in the treatment of brain tumors based on cell destruction by two mechanisms: (a) direct, by the ultrasound and (b) indirect, by an antineoplastic agent which is delivered via an ultrasonically modified BBB.

Animals↗
PubMed1990

Glutamate is the endogenous amino acid selectively released by rat hippocampal mossy fiber synaptosomes concomitantly with prodynorphin-derived peptides.

The release of endogenous amino acids from depolarized rat hippocampal mossy fiber synaptosomes was investigated to assess the possible role(s) of glutamate and aspartate in mediating the excitatory mossy fiber synaptic input. The relative proportions of prodynorphin-derived peptides concomitantly released with amino acids were also determined to further characterize the biochemical basis for mossy fiber synaptic transmission. Of the 18 amino acids shown to be present in superfusate fractions by liquid chromatographic analysis, only glutamate was released at a significantly enhanced rate from K(+)-stimulated (35 mM KCl) mossy fiber nerve endings. The rates of glutamate and aspartate release were increased by 360 +/- 27% and 54 +/- 12% over baseline, respectively. However, the K(+)-evoked release of glutamate was substantially more Ca2(+)-dependent (80%) than was the release of aspartate (49%). The veratridine (45 microM)-evoked release of both acidic amino acids was entirely blocked by 1 microM tetrodotoxin. Depolarization (45 mM KCl) also stimulated the release of the four prodynorphin (Dyn) products examined, in a rank order of Dyn B much greater than Dyn A(1-17) greater than Dyn A(1-8) much greater than Dyn A(1-13), with Dyn B efflux increasing by more than 5-fold over baseline values. These results suggest that the predominant excitatory amino acid in hippocampal mossy fiber synaptic transmission may be glutamate and that this synaptic input may be modulated by at least four different products of prodynorphin processing.

Action Potentials↗
PubMed1990

VIP-stimulated cyclic AMP accumulation in the suprachiasmatic hypothalamus.

The effect of exogenous vasoactive intestinal peptide (VIP) on the accumulation of cyclic AMP in the isolated suprachiasmatic hypothalamus prepared from rats sacrificed at midsubjective day (CT6), subjective dusk (CT12) and midsubjective night (CT18) was determined. VIP (2 microM) caused a transient, 2.5- to 3.4-fold increase in cyclic AMP which peaked 5 minutes after the addition of VIP and returned to prestimulation values by 10 minutes. The effect of VIP was dose dependent between 0.1 and 10 microM. Cyclic AMP content, both before and after VIP stimulation, was slightly but significantly higher in rats sacrificed at CT6 compared to those sacrificed at CT18; however, the degree of stimulation was similar at all three circadian times.

Animals↗
PubMed1989

Detection of zinc in isolated nerve terminals using a modified Timm's sulfide-silver method.

An ultrastructural method for detecting the presence of zinc in isolated nerve terminals from the mammalian brain is described. This method is based on the well-known Timm's sulfide-silver technique that has been used by many investigators to detect and localize zinc-containing pathways in sections of intact brain tissue. We report here a modification of this technique that we have used to assess the homogeneity, at the electron microscopic level, of a zinc-enriched synaptosomal fraction from the rat hippocampus. This technique allows biochemical assays to be performed on samples of the same tissue if desired, and also provides the large amounts of tissue needed for synaptosomal isolation. Results indicated that all of the mossy fiber synaptosomes, identified on the basis of their large size and characteristic morphology, stained for zinc using this method, as did about one-third of the smaller synaptic profiles present in the same fraction. The method described here should be useful for determining zinc retention and localization in isolated synaptosomes from other regions of the mammalian central nervous system.

Animals↗
PubMed1989

Light increases Fos-related protein immunoreactivity in the rat suprachiasmatic nuclei.

Fifteen minutes of bright, white light exposure at midsubjective night resulted in a marked increase in both the number and stain density of c-fos protein (Fos)-immunoreactive neurons in the suprachiasmatic nuclei (SCN). In all cells, peroxidase reaction product was confined to the nucleus. Most Fos-immunoreactive cells were concentrated in the ventrolateral third of the SCN, although a few immunoreactive cells were also observed diffusely distributed along the dorsal border of the nucleus and into the surrounding hypothalamus. Along the rostro-caudal extent of the SCN, the greatest density of Fos-immunoreactive cells was found at a level approximately 300-400 microns caudal of the rostral pole of the nucleus. The population of Fos-immunoreactive cells in the SCN lies within the terminal fields of both the retinohypothalamic and geniculohypothalamic tracts. In addition, a few Fos-immunoreactive cells were observed in the ventral lateral geniculate nucleus. The results suggest that retinal illumination induces transsynaptic c-fos expression in a select population of SCN neurons.

Animals↗
PubMed1989

ATP release, adenosine formation, and modulation of dynorphin and glutamic acid release by adenosine analogues in rat hippocampal mossy fiber synaptosomes.

Using a hippocampal subcellular fraction enriched in mossy fiber synaptosomes, evidence was obtained indicating that adenosine derived from a presynaptic pool of ATP may modulate the release of prodynorphin-derived peptides. and glutamic acid from mossy fiber terminals. Synaptosomal ATP was released in a Ca2+-dependent manner by K+-induced depolarization. The rapid hydrolysis of extracellular [14C]ATP in the presence of intact mossy fiber synaptosomes resulted in the production of [14C]adenosine. Micromolar concentrations of a stable adenosine analogue, 2-chloroadenosine, inhibited the K+-stimulated release of both dynorphin B and dynorphin A(1-8). 2-Chloroadenosine failed to suppress the evoked release of glutamic acid, measured in these same superfusates, unless the mossy fiber synaptosomes were pretreated with D-aspartic acid to deplete the cytosolic, Ca2+-independent, pool of this acidic amino acid. In synaptosomes pretreated in this manner, release of the remaining Ca2+-dependent pool of glutamic acid was significantly inhibited by NiCl2, 2-chloroadenosine, 5'-N-ethylcarboxamidoadenosine, cyclohexyladenosine, and R(-)-N6(2-phenylisopropyl)adenosine, but not by ATP. 2-Chloroadenosine-induced inhibition was reversed when the external CaCl2 concentration was raised from 1.8 mM to 6 mM. 8-Phenyltheophylline, an adenosine receptor antagonist, effectively blocked the inhibitory effects of 2-chloroadenosine on mossy fiber synaptosomes and significantly enhanced the K+-evoked release of both glutamic acid and dynorphin A(1-8) when added alone to the superfusion medium. These results support the proposition that depolarized hippocampal mossy fiber synaptosomes release endogenous ATP and are capable of forming adenosine from extracellular ATP, and that endogenous adenosine may act at a presynaptic site to inhibit the further release of glutamic acid and the prodynorphin-derived peptides.

Adenosine↗
PubMed1988

Glutamate and dynorphin release from a subcellular fraction enriched in hippocampal mossy fiber synaptosomes.

A procedure is described for the isolation of intact hippocampal mossy fiber synaptosomes. Electron microscopic examination revealed numerous synaptosomal profiles which are clearly of mossy fiber origin, indicated by their large size (2-6 micron diameter) and characteristic morphology. Furthermore, this fraction is enriched in zinc and dynorphin B which appear to be concentrated in mossy fiber terminals in vivo. Synaptosomes isolated by this procedure accumulated 2-deoxyglucose and retained 88% of total lactate dehydrogenase activity after incubation at 30 degrees C for 60 minutes, indicating a high degree of membrane integrity. Oxygen consumption was stimulated 4-fold by veratridine (0.1 mM) and inhibited 90% by ouabain (1 mM), suggesting that synaptosomal metabolism remained tightly coupled to ouabain-sensitive ATPase activity. Potassium-stimulated (45 mM) release of dynorphin B was completely dependent upon the presence of extrasynaptosomal calcium, while only 30% of the evoked release of glutamate was calcium-dependent. D-aspartate, which exchanges glutamate out of the cytoplasmic pool, virtually eliminated the calcium-independent component of glutamate release. This synaptosomal preparation will be useful in identifying the factors that modulate the release of amino acid and opioid neurotransmitters from hippocampal nerve terminals and in the investigation of their presynaptic mechanisms of action.

Animals↗
PubMed1988

Relationship between prostaglandin synthesis and release of acidic amino acid neurotransmitters.

The importance of glutamate as an excitatory neurotransmitter in the central nervous system has become increasingly clear. However, the presynaptic mechanism of amino acid neurotransmitter release does not appear to be consistent with existing models. A major source of controversy has been the finding that a significant amount of the amino acid release evoked by membrane depolarization is calcium-independent. It is often implied that this component of release is of cytosolic origin and subserves no neurotransmitter function. In this report, an alternative model is presented which suggests that the depolarization-induced movement of calcium ions into the nerve terminal acts, simultaneously, to stimulate the release of acidic amino acid neurotransmitters from both a vesicular and cytosolic compartment. According to this model, the influx of calcium may indirectly stimulate the biosynthesis of prostaglandins which lower the plasma membrane potential and reverse the electrogenic transport of amino acids to cause a net efflux. Such a mechanism could explain how the vesicular and cytosolic neurotransmitter pools of release are functionally related.

Animals↗
PubMed1986

Evidence for developmental synaptic regression of cholinergic afferents to the rat main olfactory bulb.

Choline acetyltransferase (ChAT) activity, acetylcholinesterase (AChE) activity and muscarinic cholinergic receptor binding were determined in homogenates of olfactory bulbs from rats killed at intervals from 4 days before through 60 days after birth. In addition, the localization of muscarinic receptors was determined using an in vitro autoradiographic technique in 6-millimicrons thick coronal sections of olfactory bulbs from rats killed at similar intervals after birth. All 3 cholinergic parameters were present in measurable quantities at birth and showed substantial increases between 1 and 20 days after birth. The most rapid increase in cholinergic parameters occurred between days 10 and 20 after birth. ChAT activity and muscarinic receptor binding decreased between days 20 and 35 and increased again between postnatal days 35 and 60. A similar developmental pattern was observed for autoradiographic grain density overlying the granule cell layer of the neonatal bulb. These data suggest that (1) centrifugal cholinergic afferents are present in the rat olfactory bulb at birth, (2) during the early postnatal period (between 10 and 20 days) synaptogenesis occurs resulting in an overproduction of cholinergic synapses and (3) between postnatal days 20 and 35, a period of synaptic reorganization occurs characterized by substantial regression.

Acetylcholinesterase↗
PubMed1986

Testosterone stimulates pituitary and serum FSH in GnRH antagonist-suppressed rats.

Thirty days of continuous treatment of adult male rats with 35 micrograms/day of the potent GnRH antagonist, (N-Ac-D-Nal (2)1, D-pCl-Phe2, D-Trp3, D-hArg (Et2)6, D-Ala10)-GnRH (RS-68439) reduced serum FSH to values below the limit of detection of the assay. Testosterone supplementation in the form of subcutaneous testosterone-filled silastic capsule implants present during an additional 30 days of GnRH antagonist administration restored serum FSH to values comparable to those observed after vehicle treatment. Pituitary FSH content, which was substantially reduced after GnRH antagonist treatment, was completely restored after concurrent testosterone supplementation. These results show that, under conditions of GnRH receptor blockade, testosterone is capable of stimulating pituitary and serum FSH in adult male rats.

Animals↗
PubMed1986

Testosterone maintains pituitary and serum FSH and spermatogenesis in gonadotrophin-releasing hormone antagonist-suppressed rats.

Groups of adult male rats were treated continuously for 30 days with either vehicle or the potent gonadotrophin-releasing hormone (GnRH) antagonist. (N-Ac-D-Nal(2)1,D-pCl-Phe2,D-Trp3,D-hArg(Et2)6,D-Ala10 )- GnRH (RS 68439; 35 micrograms/day). In addition, groups of vehicle- and antagonist-treated rats received s.c. testosterone implants sufficient to maintain serum testosterone concentrations 3.5- to 5-fold higher than those of vehicle-treated control rats. After 30 days of antagonist treatment serum LH, FSH and testosterone concentrations were at or below the detection limits of their respective assays and pituitary FSH content and GnRH receptor binding were reduced, relative to control animals, by 77 and 98% respectively. Testis weight in antagonist-treated rats was reduced by 75% and spermatogenesis was suppressed to an extent comparable to that observed in hypophysectomized rats. Testosterone, which caused a 40% reduction in serum FSH relative to control animals, prevented the antagonist-induced fall in both serum and pituitary FSH, but not GnRH receptors, below that observed in the vehicle plus testosterone-treated group. Furthermore, spermatogenesis in the antagonist plus testosterone-treated group was indistinguishable from that observed in control animals. It is concluded that testosterone is capable of maintaining serum and pituitary FSH levels in vivo, under conditions which presumably render the pituitary insensitive to hypothalamic GnRH.

Animals↗
PubMed1984

Learned helplessness: effects on brain monoamines and the pituitary-gonadal axis.

The effect of the learned helplessness paradigm, a model of depression, on biogenic amines in eight brain regions, and on the serum levels of luteinizing hormone, corticosterone, and testosterone in male rats was determined. Rats which were exposed to uncontrollable and unpredictable shocks (HY-rats) had hormone levels similar to those in appropriate control animals. However, HY-rats had higher levels of 5-HIAA in the pons/medulla oblongata and lower levels of 5-HT in the cortex than rats which could escape the shocks (HE-rats). Furthermore, striatal levels of NE were higher in HY-rats when compared to HE-rats and non-shocked controls (HC-rats). Shock treatment per se resulted in lower NE levels in the hippocampus. These data implicate the serotonergic and noradrenergic systems as possible mediators of the learned helplessness phenomenon, but do not support the view that this behavior is associated with impaired pituitary-gonadal function.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1984

Activity-wheel stress: effects on brain monoamines and the pituitary-gonadal axis.

The study was designed to study the possible role of dopaminergic and serotonergic systems in pituitary gonadal function following activity stress. 3 groups of rats were caged individually. 1 group of rats (AW) was fed for 1 h each day and had free access to a running wheel. The 2nd group (FC) had no access to a running wheel and received exactly the same amount of food as animals of the AW group. The 3rd group (UC) had 24-hour access to food. AW rats showed a gradual increase in running activity which accompanied a gradual decrease in food consumption. AW rats were sacrificed, together with their matched FC counterpart and a UC control rat, on the day that their food consumption fell below 4 g/day. The brains were dissected into nine specific areas and subjected to analysis for dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), serotonin (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) using high-performance liquid chromatography with electrochemical detection. Trunk blood was analyzed for serum levels of testosterone (T), luteinizing hormone (LH) and corticosterone (C), and weights of tests and epididymides were recorded. Both activity stress and restricted food intake resulted in a fall in serum levels of T and LH. Elevated levels were found for DOPAC in the hippocampus, hypothalamus, brainstem, and midbrain of activity-stressed rats. Levels of 5-HT and 5-HIAA were lower in the hypothalamus of these animals. Data analysis suggests a dopaminergic and serotonergic involvement in pituitary-gonadal function under activity-stress conditions.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1984

Activity wheel stress: changes in brain norepinephrine turnover and the occurrence of gastric lesions.

The effects of activity wheel stress on brain regional norepinephrine (NE) and 3-methoxy-4-hydroxyphenylglycol (MHPG) content and on the occurrence of gastric lesions were investigated. Multiple gastric lesions were present in the stomachs of all rats exposed to activity wheel stress (AW). No gastric lesions were observed in any of the food consumption (FC) or untreated (UC) control rats. In AW rats, NE levels were significantly different (p less than 0.05) from UC and/or FC rats in the hypothalamus (-25%), striatum (+120%), and hippocampus (-25%). MHPG levels in AW rats were significantly (p less than 0.05) elevated in the hypothalamus (130%), thalamus (95%), neocortex (80%), midbrain (160%), pons medulla (30%), and cerebellum (100%), indicating increased NE turnover in these brain regions. The data are discussed in terms of a possible role for brain NE in the mediation of activity stress-induced gastric lesions.

Animals↗
PubMed1983

Loss of Purkinje cell-associated benzodiazepine receptors spares a high affinity subpopulation: a study with pcd mutant mice.

In order to identify the relative number of benzodiazepine (BZ) receptors in Purkinje and granule cells, the Purkinje cell degeneration (pcd) mutant mouse was used at different ages. In these mice, Purkinje cells have degenerated almost completely by 45-50 days of age. Granule cell loss occurs only later, and is most severe between 180 and 300 days. [3H]Flunitrazepam (FNZ) and [3H]ethyl-carboline-3-carboxylate (beta-CC) were used as ligands. In the 45-50-day-old pcd mice, it was found that there is approximately a 50% decrease in the number of receptors as labeled by [3H]beta-CC or [3H]FNZ, when the binding is expressed as fmol/cerebellum. The binding decreased by approximately 80% in 300-day-old pcd mice (fmol/cerebellum). [3H]FNZ was not displaced by 1 microM RO5-4864, ruling out binding to glial cells. Nonlinear regression analysis of FNZ saturation data provided evidence for two populations of receptors (high and low affinity sites). Only the low-affinity sites were reduced in number at 45 days. [3H]beta-CC saturation data showed, however, only one population of receptors. The total number of receptors (Bmax) was significantly lower for beta-CC than for FNZ in the control mice. It appears that 50% of the total BZ receptors is associated with Purkinje cells. In addition, our data on 300-day-old pcd mutants strongly suggest the existence of granule cell-associated BZ receptors.

Animals↗
PubMed1983

Specific binding of the muscarinic antagonist [3H]quinuclidinyl benzilate is not associated with preganglionic motor neurons in the dorsal motor nucleus of the vagus.

The present study evaluates the binding of [3H]quinuclidinyl benzilate, [3H]QNB, as a measure of cholinergic muscarinic binding in six areas of the rat medulla oblongata associated with the cranial nerves. In an experimental group, the right vagus nerve was severed in the neck in order to determine whether the specific muscarinic binding sites might be located on cells that contribute efferent fibers to the vagus nerve. The level of activity of choline acetyltransferase (ChAT) also was determined in the same six areas. Additional experiments utilizing the retrograde transport of toxic ricin, a 60,000 dalton agglutinin that acts as a potent ribosomal toxin, was carried out to further evaluate localization of specific muscarinic binding in the DMN after destruction of the preganglionic efferent cells. These results support the conclusion that specific binding of the muscarinic antagonist [3H]QNB observed in the DMN of the vagus of the rat is not associated with the large cells that contribute efferent fibers into the vagus nerve. We suggest that the specific cholinergic muscarinic binding is located on interneuronal cell surfaces, on afferent terminals of local circuit neurons, or on afferent terminals of long projection axons which arise from neurons in the brainstem, hypothalamus, or forebrain.

Acetylcholinesterase↗
PubMed1982

Distribution and uptake of glycine, glutamate and gamma-aminobutyric acid in the vagal nuclei and eight other regions of the rat medulla oblongata.

In order to study the central neurochemical control of the vagus nerve, the contents of glycine, GABA, glutamate and five other amino acids have been measured in ten anatomically distinct regions of the rat medulla oblongata. Additionally, the high affinity uptake of glycine, GABA, glutamate, and leucine were measured in the same ten medullary regions. The data support published evidence for glutamatergic and GABAergic transmission in the nucleus of the tractus solitarius (NTS), and glycinergic inhibition in the hypoglossal nucleus. The data also lead to the suggestion that GABA and glutamate may be taken up into glial cells which exist along fiber tracts.

Afferent Pathways↗